1. Field of the Invention
The present invention relates to a multiple-bit transistor, a semiconductor memory using the same, and a method of driving a multiple-bit transistor. More particularly, the present invention relates to a technology useful for a semiconductor memory having storage cells each storing multiple bits.
2. Description of the Background Art
Today, nonvolatile memories including EEPROMs (Electrically Erasable Programmable Read Only Memories) are widely applied to, e.g. mobile telephones. An EEPROM, for example, usually allows only one bit of information to be stored in each storage cell transistor. However, to promote size reduction of the device, there should preferably be implemented the multiple-bit configuration of a cell transistor that allows two or more bits of information to be stored in the cell transistor.
FIG. 26 of the drawings shows a storage cell transistor with a multiple-bit configuration taught in U.S. Pat. No. 6,011,725 by way of example. As shown, the cell transistor, generally 1, has a so-called MONOS (Metal Oxide Nitride Oxide Semiconductor) structure made up of a control gate electrode (metal) 7, a silicon oxide layer (oxide) 6, a silicon nitride layer (nitride) 5, a silicon oxide layer (oxide) 4, and a P type silicon substrate (semiconductor) 2 in the order.
In the cell transistor 1, N type source/drain regions 3 and 8 each selectively become a source or a drain electrode at various stages of a write-in or a read-out sequence. Stated another way, it is indefinite which of the source/drain regions 3 and 8 functions as a source or a drain electrode. In the following description, one of the source/drain regions 3 and 8 that discharges an electric carrier, which may be electrons in this specific case, and the other region will be referred to as a source and a drain region, respectively.
FIG. 27A demonstrates how data is written to the storage cell transistor 1. As shown, the source region 8 is grounded while suitable positive voltages VD1 and VG1 are applied to the drain region 3 and control gate 7, respectively. In this condition, an electric field is established between the source region 8 and the drain region 3 and accelerates electrons, so that hot electrons are generated in the vicinity of the drain region 3. The hot electrons thus generated are injected into the silicon nitride layer 5 over the energy barrier of the silicon oxide layer 4 due to the collision thereof against phonons and the positive potential of the control gate electrode 7. Because the silicon nitride layer 5 is not electrically conductive, the hot electrons injected into the silicon nitride layer 5 localize in the vicinity of the drain region 3, forming a right bit 9a of information stored. This condition is representative of a stored-bit state (1, 0).
FIG. 27B shows a condition wherein the source and drain voltages of FIG. 27A are replaced with each other. As shown, the hot electrons injected into the silicon nitride layer 5 localize in the vicinity of the drain region 8, forming a left bit 9b of information stored. This sets up a storage state (0, 1).
FIGS. 28A through 28D show four different logical storage states available with the cell transistor 1. As shown in FIG. 28A, when electrons are not stored in either one of the right and left positions, a state (1, 1) is set up. As shown in FIG. 28D, when electrons are stored in both of the right and left bit positions, a state (0, 0) is set up. In this manner, the cell transistor 1 allows two-bit data to be stored therein. However, this data writing sequence is undesirable because the hot electrons cannot be injected into the silicon nitride layer 5 unless the voltage VG1 applied to the control gate 7 is high.
More specifically, for the injection of hot electrons, it is necessary to tunnel hot electrons from the conduction band of the silicon substrate 2 to the conduction band of the silicon oxide layer 4. An energy difference between those two conduction bands is about 3.2 electron volts (eV). However, the hot electrons lose energy on colliding against phonons present in the silicon substrate 2 and cannot be tunneled between the two conduction bands mentioned above even if a voltage of 3.2 V is applied to the control gate 7. In practice, therefore, the voltage VG1 applied to the control gate 7 must be as high as 12 V to 13 V.
While the above-stated high voltage is expected to be applied to the control gate 7 from a highly voltage-resistant transistor included in a decoder circuit, not shown, such a transistor cannot be miniaturized because miniaturization would cause punch-through to occur between the source and drain electrodes of the transistor. There is therefore a problem that it is impossible with the prior art structure described above to reduce the chip size of the entire EEPROM including the decoder circuit.
On the other hand, to read out the data from the cell transistor 1, the voltages applied to the source region 8 and drain region 3 are replaced with each other from write-in condition to measure a drain current while each drain current measured is compared with a reference current value, as will be described more specifically hereinafter. In the state (0, 0) shown in FIG. 28D, electrons localize at both of the right and left bit positions, so that the potential of the silicon nitride layer 5 is lowest among the four states. Consequently, the threshold voltage of the cell transistor 1 becomes highest and causes substantially no drain current to flow. The value of the drain current remains the same even when the voltages applied to the source region 8 and drain region 3 are replaced, and is almost zero. As a result, the two drain currents sequentially measured both are determined to be greater than the reference current.
In the state (1, 1) shown in FIG. 31A, electrons are absent from both of the right and left bit positions 9a and 9b, so that the potential of the silicon nitride layer 5 is highest among the four states. Therefore, the threshold voltage of the transistor 1 becomes lowest among the four states, causing the greatest drain current to flow. The value of the drain current remains the same even when the source region 8 and drain region 3 are replaced with each other, and is greatest among the four states. As a result, the drain currents measured one after the other are both determined to be greater than the reference current.
On the other hand, in the states (1, 0) and (0, 1) shown in FIGS. 28B and 28C, respectively, electrons localize at only one of the right and left bit positions, making the cell transistor 1 asymmetrical in the right-and-left direction with respect to potential distribution. The drain currents sequentially measured are different from each other when the voltages applied to the source region 8 and drain region 3 are replaced. It is therefore possible to distinguish the states (1, 0) and (0, 1) by determining which of the two drain currents is greater or smaller than the reference current.
The data reading sequence described above has a drawback that the current window for distinguishing the drain currents is smaller when the state (1, 0) or (0, 1) is sensed. A current window refers to a difference between the two drain currents measured by replacing the voltages applied to the source and drain regions 3 and 8 in the event of sensing the states (1, 0) and (0, 1). The current window definitely opens when electrons distinctly localize at the right or left end of the silicon nitride layer 5, i.e. when the cell transistor 1 is clearly asymmetrical in the right-and-left direction in potential or electron distribution.
Asymmetry, however, does not clearly appear in the cell transistor 1 because electrons are distributed in the silicon nitride layer 5 over some breadth. Particularly, when a gate length L, see FIG. 27A, is reduced for reducing the cell size, it is not clear at which of the right and left bit positions electrons localize, further reducing the asymmetry of the cell transistor 1 and therefore the current window. Such a small current window reduces the margins of the drain and reference currents and thereby aggravates incorrect identification of written data.
Another problem with the conventional transistor 1 is that resistance to inter-band tunneling is low, as will be described hereinafter with reference to FIG. 29. FIG. 29 shows a condition wherein the cell transistor 1 is not selected. As shown, to make the cell transistor 1 unselected, a ground potential lower than the potential assigned to read-out is applied to the control gate 7. On the other hand, the positive potential VD1 is applied to the drain electrode of a cell transistor selected. Because the positive potential VD1 is common to all of the cells in the direction of column of the memory device, it is applied to the drain region 3 of the cell transistor 1 as well.
In the condition shown in FIG. 29, a potential difference ΔV between the silicon nitride layer 5 and the drain region 3 is greater than in the case of read-out because the potential of the control gate 7 is lowered. Particularly, when electrons localize in the silicon nitride layer 5, the potential difference ΔV further increases because the electrons lower the potential of the silicon nitride layer 5. If the potential difference ΔV is great, then a tunnel current flows between the drain region 3 and the silicon nitride layer 5 and causes the silicon oxide layer 4 to deteriorate.
Moreover, a great potential difference ΔV produces a stronger electric field at the edge of the drain region 3, so that breakdown is apt to occur at the PN junction of the drain region 3 and silicon substrate 2. The breakdown causes hot holes and electrons to appear in pairs, as shown in an enlarged view in a circle 100. The hot holes 102 are attracted toward the lower potential side (the silicon nitride layer 5 side) and therefore passed through the silicon oxide layer 4, deteriorating the layer 4. The low resistance to inter-band tunneling mentioned earlier refers to the circumstances described above.
To delete data stored in the cell transistor 1, electrons stored in the silicon nitride layer 5 are withdrawn toward the drain electrode 3, as shown in FIG. 30A, or toward the control gate 7, as shown in FIG. 30B. More specifically, in FIG. 30A, a negative potential “L” and a positive potential “H” are applied to the control gate 7 and drain electrode 3, respectively, so that the electrons are withdrawn toward the drain electrode 3 higher in potential than the control gate 7. In FIG. 30B, a positive potential “H” is applied to the control gate 7 while the drain electrode 3 is grounded, so that the electrons are withdrawn toward the control gate 7 higher in potential than the drain electrode 3 and a tunnel current 104 flows.